The present invention relates to an apparatus and method for monitoring air emboli, both macro and micro, present in blood during extracorporeal circulation. The danger of air emboli in pumped hemodialysis is an ever present and serious threat to the safety of patients who must use an artificial kidney machine. With the present increased use of A-V fistulas, there is danger of the presence of air emboli resulting from greater pumping pressures necessary to maintain an adequate flow of blood.
Not only is it difficult for the person who is operating the artificial kidney to monitor visually the flow of blood, to prevent entrapped air passing through the pump, the unit or its tubing from entering the patient's body, but also, due to the high cost and relative inconvenience of hospital hemodialysis, there has been an increased usage of home treatment units, where the dangers and effects of an air embolism are accordingly increased, primarily due to the lack of skilled operating personnel. Unfortunately, even in the hospital, as facilities are expanded to accommodate larger numbers of persons in need of hemodialysis, it is increasingly difficult for available skilled personnel to monitor all patients simultaneously and it would be economically impractical to provide trained personnel on a one-to-one basis with the patient. Lastly, although the operator may visually identify and remove macro air emboli, having a volume of approximately 0.5 ml, micro emboli, on the order of from 0.1 to 0.01 ml, may be expected to pass freely into the patient's body where a possible accumulation may occur before the body is able to remove the air.
Thus, the need for an apparatus or system to detect air entrapped in blood during extracorporeal circulation is manifest, primarily for the safety of the patient and secondarily to minimize the number of skilled personnel necessary to aid and assist the patients being treated. Such monitoring systems as are presently being used for the prevention of clinically significant air embolism during hemodialysis, have been designed to prevent the injection of a single, relatively large volume of air. The inability of existing monitors to detect micro air emboli is potentially hazardous and has been a basis for frequent criticism of these devices.
Several methods which have been used for the detection of air emboli in the extracorporeal tubing, a location where air is readily detectable and removable, utilize ultrasonic, photoelectric or electric sensors. These detectors are certainly better than no monitor, but are unable to perform satisfactorily under all conditions and to detect small volumes of air passage through the tubing. Criticisms of the photoelectric detectors have been reported as a result of false triggering occasioned by factors such as variations in color and optical density of the various fluids infused in the tubing during dialysis. Use of these detectors is also limited by the transparency and size of the various tubings available and their sensitivity, which often causes a false triggering with changes in the light intensity within a room.
Another disadvantage has been the necessity of disarming the sensor mechanism during various periods of treatment such as saline infusion, and, when the system has been disarmed, it must subsequently be reset and usually, with time-consuming adjustments.
Methods for monitoring electrical impedence of the blood lines also exist, however, these have undesirably required invasion of the blood stream with electrodes. At least one known system is able to detect air utilizing electrical impedance changes without direct contact with the flow of blood. By placing adjacent electrodes around the tubing, a capacitor is constructed whose dielectric is formed by the wall and the contents therein. Any fluid, conductive or nonconductive, increases the capacitance as its relative dielectric constant is much higher than that of air. Therefore, any air passing through the tubing as a bubble, induces between the electrodes a change in capacitance which is readily detected.
Notwithstanding the various devices which are available for monitoring air emboli and are presently being used, none has been found to possess the high sensitivity necessary to detect micro emboli without attendant, undesirable oversensitivity to external conditions, e.g., heat, light and movement. Moreover, frequent adjustments are often necessitated during operations of the devices or the equipment with which they are used to avoid erroneous indications. Finally, in order to avoid frequent responses to the micro emboli which some systems might be capable of detecting and which individually may not be harmful, the minimum size may be increased by the operator who then does not know the accumulated quantity of these emboli having passed into the patient within a given period of time.